Add lecture on the Solow--Swan model

[jstac]

I've kicked this off by adding md file in-work/solow.md

This file needs to be cleaned up and edited

• check that code runs, clean up
• make sure that all maths displays properly
• add suitable links for people, definitions, etc. (e.g., Robert Solow, fixed point)
• add more interesting discussion
• add exercises

[Smit-create]

Here is the PR: #42

[Smit-create]

This can be closed?

[jstac]

Yes, thanks for great work @Smit-create